Actually Bayern II has an own supporter base that really watch every match in Munich if possible. I do not know how many of them follow when they play away in the west of Germany - and there some of the "normal" FC Bayern supporters in the West might come, too, to watch the talents.
I still live in Bavaria - but we have 350 kilometre distance to Munich, but only 35 to Frankfurt. My neighbour is Hamburg fan since childhood. He got fan in the 60s. There was never a question that I want to be a Bayern fan - and I do not even remember the successes in the 70s as I just was too small then. My first real memories are from mid to end of the 70s - a rather difficult Bayern period. We have a big Eintracht fan crowd here - a lot travel to every match - and a lot Offenbach fans, too. There is some Gladbach fans that got hooked in the 70s - and since their success period in the 90s (or around 2010) some BVB fans.
There have to be 2 leagues distance between first and second team. A U23-team of a 2nd Bundesliga team cannot play higher than in one of the Regionalligas. But right now only Bayern's U23 team plays 3rd league. With only three teams being able to get promoted from one of the five Regionalligas - it was pretty difficult for Bayern to get the promotion back to 3rd league after going down in 2012 or 2013. In 2015 or 2016 - when Erik Ten Hag was coach of Bayern's U23 they failed in the last minute of the relegation match because of a goalkeeper mistake. I think that the quality differences between the 2nd and 3rd league aren't huge - but the differences between the 5 Regionalliga legs and the 3rd league are.
It is pretty difficult to keep a U23 team in the 3rd league as you always have to implement new young players from the U19 into the team - whereas the best talents that use that team as a stepstone into professional football leave after one year.